# Full text of "Handbook Of Chemical Engineering - I"

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450
CHEMICAL ENGINEERING
temperatures absorption glasses, St Fig. 20, are used between the lamp and objective, or in front of the objective, 'to diminish the observed intensity of the source. The relation between the temperature of the source tf ° absolute and the observed temperature #0° absolute measured with the absorption glass interposed is as follows:
1       1       A
-  —-  = A
#      tfo where A is for most practical purposes a constant.
Usually the instrument is furnished with a table showing the relation between the current through the lamp and the temperature both with and without the absorption glass. If, however, this relation is not given when the absorption glass is used, it may be readily determined by measuring the constant Am the above formula. To do this, sight without the absorption glass on a muffle furnace or any uniformly heated furnace at 1,200 to 1,500°C. and observe the temperature #, in degrees absolute. Then with the absorption glass in place, match the filament again and observe to what temperature, #0, in degrees absolute the current through the lamp corresponds. The difference in the reciprocal of these two temperatures is the constant A, which is usually of the order of magnitude —0.0002. This determination should be repeated several times and at several different temperatures of the furnace. The separate values of A should not differ by more than 1 per cent and the mean value is used for computing the relation between the observed absolute temperature with the absorption glass and the true temperature of the source. In making these computations care must be exercised that all temperatures are converted to degrees absolute. The following table illustrates the calibration of a certain pyrometer, both with and without the absorption glass. The constant of the absorption glass has the value A — 0.000280. By use of this glass temperatures as high as 2,730°C. can be measured although the temperature of the lamp does not exceed 1,360°C.
TABLE 11
vvmreiiij, amperes	Without absorption glass	With absorption glass
0.26	634	943
0.28	765	1,190
0.30	860	1,386
0.32	936	1,555
0.34	1,002	1,710
0.36	1,060	1,854
0.38	1,113	1,992
0.42	1,201	2,237
0.46	1,281	2,478
0.50	1,359	2,733
It will be noted that the range of currents required is small, from in general about 0.3 to 0.6 amp. Thus if the ammeter is designed to give full-scale deflection on 0.6 amp. nearly half of the scale, from 0 to 0.26 amp. is never used. The Hickok depressed zero ammeter now furnished with the L. and N. pyrometer meets this objection. The moving-coil system including the supports, pivots and pointer may be adjusted relative to the magnet, by turning a lever on the case of the instrument, to one of two positions. In one position the pointer is adjusted on open circuit so that it falls over the first graduation on the scale. This adjustment is similar to the ordinary zero